1. Field of the Invention
The technology disclosed relates to a controller and a control method for a motorised vehicle, and in particular to the control of a motorised vehicle having at least a left wheel and a right wheel, the left wheel being driven by a left motor and the right wheel being driven by a right motor. In accordance with such motorised vehicles, linear motion of the vehicle is typically effected by driving the left motor and the right motor in the same direction, whilst spinning of the vehicle is typically effected by driving the left motor and the right motor in opposite directions. A typical example of such a motorised vehicle is an electric wheelchair.
2. Description of the Prior Art
Motion of a motorised vehicle such as an electric wheelchair is typically effected by a left and right wheel which can be driven independently. Substantially forward and reverse motion of the vehicle is produced by driving the left and right wheel in the same direction, whilst turning of the vehicle is achieved by driving the left and right wheel at different speeds. Indeed the vehicle may even be caused to spin without substantial linear motion by driving the left and right wheel in opposite directions.
It is known to drive such left and right wheels of a motorised vehicle by means of a motor, such as an electric motor, connected to each wheel. Control circuitry is typically provided to respond to command signals from a user interface (such as a joystick) and to cause appropriate voltages to be applied to each motor to bring about the motion requested by the user.
It is further known that the performance of an electric motor, in particular the speed at which it causes an associated wheel to rotate, depends not only on the voltage applied to the motor, but also on the load which is experienced by that motor. When the load increases, for example when the vehicle is climbing a slope, the increased load will cause the motor to turn more slowly for a given supply voltage. In order to counteract such effects, it is known for motorised vehicles such as electric wheelchairs to comprise motor control circuitry which implements a technique known as IR compensation. According to this technique, the motor control circuitry monitors the left and right motor in terms of their instantaneous voltage and current consumption. From these the motor control circuitry can determine the motor speed (primarily determined by the applied armature voltage) and the motor torque (primarily determined by the armature current). The motor control circuitry can then identify situations in which the load on a motor increases (resulting in greater motor torque) by virtue of an increase in the armature current. The real motor speed can be estimated in accordance with the equation:Speed=kemf(Vm−(Im×Rm))
where kemf is the back e.m.f. constant, Vm is the voltage applied to the motor, Im is the current drawn by the motor and is the resistance of the motor windings (armature). A difference between this real motor speed and the requested motor speed is then calculated and a correction is applied, such that for example when the real motor speed drops due to an increase in load, the motor control circuitry compensates by increasing the voltage supplied to that motor to maintain the requested speed.
However, it has been found that such compensation based feedback mechanisms can make a motorised vehicle such as an electric wheelchair rather unstable when turning. This effect is particularly pronounced when the vehicle is a front wheel drive vehicle, as is often the case for an electric wheelchair. This turn instability for a front wheel drive vehicle results from the fact that the trailing centre of mass of a front wheel drive vehicle (consisting mainly of the weight of the user and the weight of the heavy-duty batteries which power the vehicle) causes the driving torque on the inside wheel to increase, and hence slow down. This has the effect that the radius of turn of the vehicle decreases, without the user having requested such a change in turn radius. The tighter turn radius leads to a higher centripetal force acting on the vehicle (since centripetal force is inversely proportional to turn radius) and this further increases the driving torque on the inside wheel, causing an even tighter turn radius. A true vicious circle thus results, which could have serious consequences for the user of the vehicle.
It should be noted that this turn instability problem in practice also affects straight line driving. Whilst a user of the vehicle may seek to drive in a straight line, small left or right deviations will always be present (corresponding to a very large turn radius) and any small offset in turning direction (also known as spin demand) will lead to an even tighter turn with positive feedback. The user has to manually compensate and is constantly fighting to keep the vehicle driving straight. The resulting “fish tailing” is an undesirable phenomenon for the user.
A related problem also arises for front wheel vehicles when driving on a camber. The position of the centre of mass of the vehicle behind the front wheels causes the uphill wheel load to increase, and hence slow down, turning the vehicle up the slope. This effect (known as “camber veer”) can be particularly problematic for front wheel drive wheelchairs which implement a “gate shaping algorithm”, since this tends to slow the wheelchair down as the spin demand increases, and thus only a small correction window is available to the user seeking to avoid this veering effect.
Some background technological information to the technology disclosed can be found in the “enAble40 Powerchair Control System” manual produced by Curtis Instruments, Inc. of New York, USA; in U.S. Pat. Nos. 5,033,000 and 5,307,888; and in U.S. Patent Application Publication 2010/0007299 A1.
Accordingly it would be desirable to provide an improved technique for controlling such motorised vehicles.